1. Field of the Invention
The present invention relates to an imaging device for converting an electromagnetic radiation such as visible light, infrared rays, ultraviolet rays, X-rays, etc. into an electric signal, and more particularly to an infrared imaging device for converting infrared rays into an electric signal.
2. Description of the Related Art
Infrared imaging devices are classified into a quantum type which detects incident infrared rays with a photodiode or the like and a thermal type which converts an increase in temperature of a structural body due to incident infrared rays into an electric signal with a thermoelectric transducer. Both types of infrared imaging devices are used to measure a temperature distribution over the surface of a subject to be imaged, for example.
One conventional infrared imaging device is disclosed in Japanese patent application No. 098009/96, for example, which is an earlier invention by the inventor of the present invention. FIGS. 1 and 2 of the accompanying drawings are a cross-sectional view and a circuit diagram, respectively, of the disclosed conventional infrared imaging device. The conventional infrared imaging device is a thermal-type infrared imaging device. As shown in FIG. 1, the infrared imaging device has a semiconductor substrate 20, a scanning circuit 21 on the surface of the semiconductor substrate 20, and a photodetector on the scanning circuit 21 for converting incident infrared rays into an electric signal. The scanning circuit 21 and the photodetector comprise an integrated matrix of pixels for generating a signal representing a two-dimensional infrared image. The photodetector comprises an infrared absorbing layer 29 for absorbing infrared rays, a diaphragm (silicon oxide film) 28 for preventing heat from being dissipated away, and a thermoelectric transducer 27 for converting heat into an electric signal.
The diaphragm 28 has its lower layer removed by etching, so that it is of a floating film-like structure. The thermoelectric transducer 27 comprises a bolometer whose electric resistance varies depending on the temperature, the bolometer being made of titanium. An infrared ray applied to each of the pixels is absorbed by the infrared absorbing layer 29 at each pixel, increasing the temperature of the diaphragm 28 at each pixel. The increase in the temperature is converted into an electric signal by the titanium bolometer. Electric signals generated by the respective pixels are successively read by the scanning circuit 21.
The infrared imaging device also has a silicon oxide film 22, cavities 23, ground lines 24, signal lines 25, and vertical selection lines 30.
As shown in FIG. 2, the scanning circuit 21 of the infrared imaging device has source followers 907, 912, load transistors 913, horizontal switches 909, 916, horizontal signal lines 911, NPN transistors 902, PNP transistors 904, integrating capacitors 905, ramp waveform generators 915, pixel switches 920, horizontal signal lines 918, titanium bolometers 901, 903, level converters 921, 922, 923, 924, 925, 926, 927, 927 for being supplied respectively with horizontal data 929, a horizontal clock 930, an S/H pulse 931, a reset pulse 932, horizontal data 933, a horizontal clock 934, vertical data 935, and a vertical clock 936, a horizontal shift register 910 for outputting horizontal pulses I1-I5, a horizontal shift register 917 for outputting horizontal selection signals H1-H128, and a vertical shift register 919 for outputting vertical selection signals V1-V128.
In FIG. 2, each of the titanium bolometers 901 is disposed on the corresponding diaphragm 28, and is sensitive to incident infrared rays. When a voltage Vb1 is applied to the base of an NPN transistor 902, a voltage (Vb1xe2x88x92VBE) is applied to the titanium bolometer 901 where VBE represents a base-to-emitter voltage of the NPN transistor 902. If the titanium bolometer 901 has a resistance Rb1, then a current Ic1=(Vb1xe2x88x92VBE)/Rb1 flows through the collector of the NPN transistor 902.
The titanium bolometers 903 are disposed on the semiconductor substrate 20, and hence are not sensitive to incident infrared rays. This is because the titanium bolometers 903 are used as a reference with respect to the titanium bolometers 901. When a voltage Vb2 is applied to the base of an NPN transistor 904, a current Ic2=(Vb2xe2x88x92VBE)/Rb2 flows through the collector of the NPN transistor 904 where Rb2 represents the resistance of the titanium bolometer 903.
When no incident infrared ray is applied, the currents Ic1, Ic2 are in equilibrium with each other, and almost no current flows in the integrating capacitor 905. When an incident infrared ray is applied, the temperature of the thermally isolated diaphragm 28 rises, changing the resistance of the titanium bolometer 901 on the diaphragm 28. The change in the resistance of the titanium bolometer 901 changes the current Ic1. Since the resistance of the titanium bolometer 903 on the semiconductor substrate 20 does not change, the current Ic2 does not change. Because of the changing current Ic1, there is developed a current difference xcex94I=(Ic2xe2x88x92Ic1) which is stored in the integrating capacitor 905. The current difference xcex94I comprises a signal component and a bias component which cannot be removed, with a larger bias component being removed.
Another conventional imaging device is an amplification-type solid-state imaging device as disclosed in Japanese laid-open patent publication No. 289381/89, for example. The amplification-type solid-state imaging device disclosed has a photodiode and a current mirror that are combined with each other for reducing the effects of the threshold voltage VT and parasitic capacitance of an amplifying element.
Japanese laid-open patent publication No. 78218/94 reveals an imaging device in which the difference between an output signal produced by a pixel when a reset time is long and an output signal produced by the pixel when the reset time is short is determined to remove fixed pattern noise (FPN).
In an imaging device disclosed in Japanese laid-open patent publication No. 242330/96, the difference between an output signal produced by a pixel immediately before the signal is reset and an output signal produced by the pixel immediately after the signal reset is determined to correct signal variations of a reading circuit.
The imaging device shown in Japanese laid-open patent publication No. 098009/96 is capable of cutting off a larger bias component and extracting a signal component, but cannot increase the amplification for signals if there are large variations between the pixels.
In imaging devices composed of a plurality of pixels, there are usually variations between the pixels. These variations between the pixels may be caused by variation between detectors such as bolometers or variations in threshold voltages and parasitic capacitances of amplifying elements. In a bolometer-type infrared imaging device, for example, bolometer resistances vary from several percentages to several tens of percentages due to variations of the thickness of bolometer films, variations of specific resistances, and variations of patterned dimensions.
Such pixel variations may pose a serious problem in reading signals. For example, when a subject having a temperature difference of 1xc2x0 C. is imaged, the temperature of the bolometer temperature changes by about 1 mxc2x0 C., and the resistance of the bolometer changes by about 0.001% if the temperature coefficient of resistance of the bolometer is 1%/xc2x0 C. In order to read such a small resistance change, it should preferably be amplified by an amplifying circuit. If there are large resistance variations between the pixels, however, the dynamic range of the amplifying circuit is limited by the large resistance variations, and the amplification factor of the amplifying circuit cannot be increased.
The amplification-type solid-state imaging devices disclosed in Japanese laid-open patent publications Nos. 289381/89, 78218/94, and 242330/96 are only effective to correct variations contained in amplifying elements, such as parasitic capacitance and threshold voltage variations, and do not correct variations of detectors themselves.
It is therefore an object of the present invention to provide an imaging device which is capable of correcting variations between pixels due to variations inherent in detectors and amplifying elements to allow signals to be amplified smoothly in an imaging unit and to be processed smoothly outside of an imaging unit.
According to the present invention, there is provided an imaging device comprising a reading circuit which includes a first regulated constant-current source for supplying a constant bias current to detectors that convert electromagnetic radiation into electric signal, and a second regulated constant-current source connected to the first regulated constant-current source, for correcting variations of the detectors.
The second regulated constant-current source is capable of correcting variations of pixels due to variations of amplifying elements and variations inherent to the detectors, with the result that the amplification factor of an amplifying circuit on a chip can be increased.
The first regulated constant-current source may comprise a bipolar transistor having an emitter connected to the detectors and a collector connected to the second regulated constant-current source, or a field-effect transistor having a source connected to the detectors and a drain connected to the second regulated constant-current source.
The second regulated constant-current source may comprise a bipolar transistor and a resistor connected to an emitter of the bipolar transistor, or a field-effect transistor and a resistor connected to a source of the field-effect transistor.
With the second regulated constant-current source, the amplification factor of the transistor is lowered to make noise of the transistor smaller in an outputted constant current.
If the resistor has the same temperature coefficient as the detectors, then the regulated constant-current circuit is the same as the temperature coefficient of the detectors, resulting in a reduction in temperature drifts.
The second regulated constant-current source may comprise a plurality of bipolar transistors and a plurality of resistors connected to emitters of the bipolar transistors, each of the resistors having a resistance inversely proportional to an area of the emitter of one of the bipolar transistors, or a plurality of field-effect transistors and a plurality of resistors connected to sources of the field-effect transistors, each of the resistors having a resistance inversely proportional to a gate length of one of the field-effect transistors. Since voltages applied to the resistors are equal and highly accurate, variations of the pixels can be corrected highly accurately.
The resistance ranges from 1 kxcexa9 to 500 kxcexa9, and preferably from 5 kxcexa9 to 100 kxcexa9. Therefore, Johnson noise can be reduced without increasing the breakdown voltage of the imaging device.
The imaging circuit may further comprise two data buffers for storing variation data of the detectors. The data buffers allow correction data to be read into the imaging device while signals of the pixels are being integrated, so that the period of time for integrating the signals can be increased to reduce noise.
The imaging circuit may further comprise means for comparing signals from pixels of the detectors with an upper or lower limit of a dynamic range of the reading circuit, means for generating variation data of the detectors based on the result of the comparison, and means for manipulating a most significant bit (MSB) of each of the variation data of the detectors to determine a value of the MSB based on the result of the comparison, and successively manipulating bits of the variation data of the detectors to determine values of the bits up to a least significant bit thereof.
With the above arrangement, it is possible to acquire correction data for correcting the variations of the pixels easily in a short period of time. This is because of the use of an algorithm for searching for bits of the correction data while monitoring the dynamic range of the signals.
The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.